7.4. Path Strings

When a program is running, there is alway a current directory (or, present working directory, pwd). Files in the present working directory can be referred to by their simple names, e.g., sample.txt, so project files can be referred to by their simple names because they are in the same directory.

Referring to files not in the current directory is more complicated. You should be aware from using the Windows Explorer or the macOS Finder that files and directories are located in a hierarchy of directories in the file system. On a Mac, the file system is unified in one hierarchy. On Windows, each storage drive has its own hierarchy.

Files are generally referred to by a chain of directories before the final name of the file desired. A path string (or simply path) is used to represent such a sequence of names. Elements of the directory chain are separated by operating system specific punctuation symbols: In Windows the separator is backslash, \, and on a Mac it is (forward) slash, /. For example, on a Mac the path

/Users/anh

starts with a /, meaning the root or top directory in the hierarchy, and Users is a subdirectory, and anh is a subdirectory of Users (in this case the home directory for the user with login anh). It is similar with Windows, except there may be a drive in the beginning, and the separator is a \, so

C:\Windows\System32

is on C: drive; Windows is a subdirectory of the root directory \, and System32 is a subdirectory of Windows. Each drive in Windows has a separate file hierarchy underneath it.

Paths starting from the root of a file system, with \ or / are called absolute paths or full paths because they begin with the root of the file hierarchy. Since there is always a current directory, it makes sense to allow a path to be relative to the current directory. In that case do not start with the slash that would indicate the root directory. For example, if the current directory is your user home directory, you likely have a subdirectory Downloads, and the Downloads directory might contain examples.zip. From the home directory, this file could be referred to as Downloads\examples.zip in Windows or Downloads/examples.zip on a Mac.

Referring to files in the current directory just by their plain file name is actually an example of using relative paths.

With relative paths, you sometimes want to move up the directory hierarchy: .. (two periods) refers to the directory one level up the chain.

For example, suppose you have a data directory under your introcscs directory, and you place your sample.txt in the data directory. When running your project from the project directory (Ch07File), you would refer to the sample.txt file to read as ..\data\sample.txt in Windows or ../data/sample.txt in macOS. Follow this one step at a time: Starting from the Ch07File project folder, where the program is running, go up one folder (..) to the introcscs folder, then down into the data folder, and refer to the sample.txt file in that folder.

Occasionally you need to refer explicitly to the current directory: It is referred to as .. (a single period).

7.4.1. Paths in C#

The differing versions of paths for Windows and a Mac are a pain to deal with. Luckily C# abstracts away the differences. It has a Path class in the System.IO namespace that provides many handy functions for dealing with paths in an operating system independent way:

For one thing, C# knows the path separator character for your operating system, Path.DirectorySeparatorChar.

More useful is the function Path.Combine, which takes any number of string parameters for sequential parts of a path, and creates a single string appropriate for the current operating system. For example, Path.Combine("bin", "Debug") will return "bin\Debug" or "bin/debug" as appropriate. Path.Combine("..", "data", "sample.txt") will return a string with characters ..\data\sample.txt or ../data/sample.txt.

Even if you know you are going to be on Windows, file paths are a problem because \ is the string escape character. To enter the Windows path solve explicitly you would need to have "..\\data\\sample.txt", or the raw string prefix, @ can come to the rescue: @"..\data\sample.txt".

Path strings are used by the directory-class and by the file-class. You can look at the Path class in the C# Language Reference for many other operations with path strings.

7.4.2. The Directory Class

The Directory class is in the System.IO namespace. Directories in the file system are referenced by Path Strings. You can look at the C# language reference for a wide variety of functions in the Directory class including ones to list all the files in a directory or to check if a path string represents an actual directory.